In commonly owned U.S. Pat. No. 3,713,106, issued 23 Jan. 1973 in the name of Giorgio Dal Monte, there has been described an intermediate switching station or exchange of this general type serving incoming and outgoing channels of a pulse-code-modulation (PCM) communication system. The term "channel", as defined in that prior patent, denotes a succession of periodically recurrent time slots interleaved with similar time slots of other channels in a predetermined sequence to form a PCM message frame. In practice, as also stated in the patent, such a frame may be composed of 32 time slots each containing eight bits for a total of 256 bits per frame, the frames following one another at a repetition rate or cadence of 8000 Hz corresponding to 2.048 Mbits per second.
With n links converging at the exchange, each link carrying m channels, a total of m.multidot.n lines have to be scanned periodically for switching signals such as area or central-office codes sent out by calling subscribers to establish connections with other subscribers reachable through the exchange. Such signals are conveniently transmitted in a special time slot reserved therefor in the PCM message frame, e.g. the 16th time slot in a 32-slot frame as described in the Dal Monte patent. Since another time slot of each frame (No. 0) is allocated to a frame-synchronizing signal, there is room in such a frame for 30 channels (m = 30).
Thus, in a sequence of m frames it is possible to transmit switching codes for all the lines served by a given trunk terminal. In practice, a cycle of 32 rather than 30 frames will generally be used for the sequential transmission of switching information from the associated group of 30 local lines. During the same cycle, similar switching information is sent to the exchange via all the other links for their respective line groups. We may refer to such local lines as subscribers even though they may be only temporarily connected to specific subscriber stations.
With the number n of links equal to 31, for example, the total number of 930 switching codes can be processed within a switching cycle of 1024 time slots if the processing time for each code is on the order of a time slot, i.e. about 4 .mu.sec in the specific instance here discussed. Such processing, as is known, involves the repetitive evaluation of switching codes from the same channel, for the purpose of verifying their authenticity with exclusion of spurious signals or transients. A switching operation should therefore be carried out only if the same signal persists on a line for a predetermined period (which may be different for different types of switching operations), i.e. if it recurs in the same time slot during a number of consecutive cycles.